1. Field of the Invention
The present invention generally relates to a light diffusion element, a screen, and an image projector.
2. Description of the Related Art
With the development of a liquid crystal technology, various image display apparatuses have been recently developed. A rear projector is one of such image display apparatuses that uses liquid crystal. In the rear projector, light emitted by a light source travels to a light modulator after passing through an illumination system. The light modulator modulates the light to form an image made of lights, and the image is displayed on a screen from a rear side by an optical system including a lens or a mirror, thereby the image is displayed on the screen. Such image projector is employed in a large-screen television set for public use and an information display and advertisement display for enterprise use.
A white light source such as a lamp is used, for example, as a light source for the image projector. A white light emitted from the white light source is separated into three primary colors, red (R), green (G), and blue (B) with regard to space and time, and each of the separated lights is modulated based on image signals by a light modulator. The modulated lights are re-combined to form a color image.
An illumination system of the image projector includes a light homogenizer that makes distributed intensity of light emitted from a light source uniform, a light deforming unit that deforms a cross-sectional shape of light flux from circle to square able to fit the light to a light modulator, a separating unit (such as a color filter) that separates a white light emitted from a light source into three primary colors, and an optical element (such as a lens and a mirror) that forms an image from light coming from a light source to display the image on a desired position with a desired size.
A reflection-type light modulator such as a digital micromirror device (DMD) (registered trademark) or a transmissive or a reflective liquid crystal panel can be used as a light modulator of the image projector. The light modulator modulates light. For example, in a three-layer method, three light modulators separate the white light into three primary colors, and each of the three light modulators independently modulates a corresponding color. In a single-chip method, one light modulator alternately separates a white light into three primary colors at a different timing by using rotating three-color filter located on a light path, and alternately modulates each of the three color lights at a different timing.
A screen of the image projector transmits an image (i.e., lights) coming from the rear face and displays the image on the front face for viewers. The screen includes a Fresnel lens that deflects the spreading light toward viewers and a lenticular lens that widen a viewing angle mainly in a horizontal direction. If a light diffusion layer is mounted on one or both of the Fresnel lens and the lenticular lens, it is possible to widen a viewing angle in a vertical direction by diffusing the light by the light diffusion layer.
In the conventional screen, it is impossible to display a clear image due to speckles (glares on an image that are called scintillation) because scattered light caused by a diffusion layer in the screen interferes with one another.
Recently, to display more vivid images, some image projectors use a laser that emits lights with wavelength bands corresponding to R, G, and B as a light source. However, when such a laser is used, speckles appear more remarkably because lights emitted from the laser are highly coherent. To obtain a clear image while using the laser, it is important to reduce speckles.
To solve the problem, Japanese Patent Application Laid-open No. 2001-100316 discloses a technology for reducing speckles by changing in terms of time at least one of a scattering distribution and a phase of waves in a light diffusion layer. Japanese Patent Application Laid-open No. 2005-352020 disclose another technology for reducing speckles by periodically applying voltage to at least two liquid crystal layers to obtain an effect of vibrating scattering surfaces of the liquid crystal layers.
However, in the former technology, the scattering distribution by the light diffusion layer is changed in terms of time so that a transmission rate (brightness of a display image) on a screen also changes depending on the light scattering character of the light diffusion layer. If this technology is applied to a single-chip projector, it is necessary to synchronize a timing of displaying R, G, and B images with a timing of changing the scattering characteristic of the light diffusion layer, thus causing controlling of the scattering characteristic of the light diffusion layer to be complicated. Unless synchronizing, a color balance of an obtained image is lost. Moreover, according to the latter technology, a plurality of liquid crystal layers causes a complicated structure of a light diffusion element and an increase of manufacturing costs.